This invention relates to single RF tuners and more particularly to a system and method for temperature compensation within single conversion tuners.
Single conversion tuner circuits are very prevalent. The first element in the frond end of the typical RF tuner is a dual gate FET amplifier. The inherent problem with these devices is that they are sensitive to temperature causing the signal level to change. Over an industrial temperature range of +85C to xe2x88x9240C, there is typically between 6 and 10 db of gain variation. One method of compensating for gain variation is to use automatic gain control (AGC) in a feed back manner. However, when gain is retarded, the noise level rises for this application by approximately between 6 and 8 db. For this application RF AGC is the only available method of retarding the system gain. As the temp goes down the RF gain goes up to counteract this action and the RF gain then needs to be reduced. Reducing the RF gain unfortunately increases the system noise figure, thereby decreasing the SNR out of the tuner. Thus, it is desired not to have to retard the RF gain setting. Also, it is desired to save the power of an AGC circuit which can be in the order of 40 mA, which does not seem like much, but when total available power is limited, and where the heat from excess power usage is important, 40 mA can be critical.
The present invention is directed to a system and method which takes advantage of the fact that the first stage dual gate in the RF input circuit of a tuner is actually quite well behaved in terms of its gain variation versus temperature. In fact, there is almost a linear degradation in gain as temperature increases. For example, assume that at xe2x88x9240C, the system gain is 40 dB. In this example, we found that gain would degrade to 32 dB with an almost linear slope at 85xc2x0 C. Using this almost linear property, I have designed an IF stage which has an opposite slope as a function of temperature and which maintains the proper linearity over the gain variation.
The circuit and method of this invention implements a thermistor in the common emitter bias of an IF amplifier tailored to precisely pinch off the bias current to reduce the gain of the IF amplifier as a function of temperature. The characteristic of the IF amplifier is selected to be opposite of the gain of the input FET so that the two gain curves exactly cancel.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.